Synthesis and Characterization of Amphiphilic Gold Nanoparticles

Summary

Amphiphilic gold nanoparticles can be used in many biological applications. A protocol to synthesize gold nanoparticles coated by a binary mixture of ligands and a detailed characterization of these particles is presented.

Abstract

Gold nanoparticles covered with a mixture of 1-octanethiol (OT) and 11-mercapto-1-undecane sulfonic acid (MUS) have been extensively studied because of their interactions with cell membranes, lipid bilayers, and viruses. The hydrophilic ligands make these particles colloidally stable in aqueous solutions and the combination with hydrophobic ligands creates an amphiphilic particle that can be loaded with hydrophobic drugs, fuse with the lipid membranes, and resist nonspecific protein adsorption. Many of these properties depend on nanoparticle size and the composition of the ligand shell. It is, therefore, crucial to have a reproducible synthetic method and reliable characterization techniques that allow the determination of nanoparticle properties and the ligand shell composition. Here, a one-phase chemical reduction, followed by a thorough purification to synthesize these nanoparticles with diameters below 5 nm, is presented. The ratio between the two ligands on the surface of the nanoparticle can be tuned through their stoichiometric ratio used during synthesis. We demonstrate how various routine techniques, such as transmission electron microscopy (TEM), nuclear magnetic resonance (NMR), thermogravimetric analysis (TGA), and ultraviolet-visible (UV-Vis) spectrometry, are combined to comprehensively characterize the physicochemical parameters of the nanoparticles.

Introduction

The ligand shell of gold nanoparticles can be engineered to exhibit several different properties that can be applied to address challenges in biomedicine^1,2,3,4. Such versatility allows for the control of the intermolecular interactions between nanoparticles and biomolecules^5,6,7. Hydrophobicity and charge play a decisive role, as well as other surface parameters that affect how nanoparticles interact with biomolecules^5,....

Protocol

1. Synthesis of 11-mercapto-1-undecanesulfonate (MUS)

NOTE: This protocol can be used at any scale desired. Here, a 10 g scale-of-product is described.

1. Sodium undec-10-enesulfonate
   1. Add 11-bromo-1-undecene (25 mL, 111.975 mmol), sodium sulfite (28.75 g, 227.92 mmol), and benzyltriethylammonium bromide (10 mg) to a mixture of 200 mL of methanol (MeOH) and 450 mL of deionized (DI) water (4:9 v/v MeOH:H₂O ratio) in a 1 L round-bottom flask.
   2. Reflux the reaction mixture at 102 °C for 48 h. Cap the system with a pressure relief mechanism—for example, a balloon with a needle....

Results

The reaction steps to synthesize MUS are shown in Figure 1. The ¹H NMR spectra of the product of each step are represented in Figure 2. The synthesis workflow of the binary MUS:OT amphiphilic gold nanoparticles is described in Figure 3. Following the synthesis, the workup of the nanoparticles consisted of washing the particles several times with ethanol and DI water. Prior to any character.......

Discussion

This protocol describes first the synthesis of MUS ligand and, then, the synthesis and characterization of amphiphilic MUS:OT gold nanoparticles. Synthesizing MUS with minimal salt content enables a better reliability of the stoichiometric ratio between the ligands during the nanoparticle synthesis, which is a key factor for the reproducible synthesis of MUS:OT nanoparticles with a target hydrophobic content (Figure 8). The use of methanol as a common solvent for MUS and OT, along with the s.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
11-bromo-1-undecene	Sigma Aldrich	467642-25 ml
Sodium Sulfite	Sigma Aldrich	S0505-250 g
Benzyltriethyl-ammonium bromide	Sigma Aldrich	147125-25 g
Methanol	VWR	BDH1135-2.5 LP
DI water	Millipore	ZRXQ003WW	Deionized water
1 L round bottom flask	DURAN	24 170 56
Diethyl ether	Sigma Aldrich	1.00930 EMD Millipore
Stirring bar	Sigma Aldrich	Z329207,
Dow Corning High Vacuum Grease	Sigma Aldrich	Z273554 ALDRICH
Filtering flask	DURAN	20 201 63
Filtering Buchner Funnel	FisherSci	11707335
Ethanol >99.8%, ACS, Reagent	VWR	2081.321DP
Deuterium dioxide	Sigma Aldrich	151882 ALDRICH
Thioacetic acid 96%	Sigma Aldrich	T30805 ALDRICH
Carbon black	Sigma Aldrich	05105-1KG
Celite	Sigma Aldrich	D3877 SIGMA-ALDRICH	Filtration medium
Condenser	Sigma Aldrich	Z531154
Hydrochloric acid, ACS reagent 37%	Sigma Aldrich	320331 SIGMA-ALDRICH
Sodium Hydroxide, BioXtra, pellets (anhydrous)	Sigma Aldrich	S8045 SIGMA-ALDRICH
Centrifuge tubes	VWR	525-0155P
250 mL round bottom flask	DURAN	24 170 37
500 mL round bottom flask	DURAN	24 170 46
Nitric acid, fACS reagent 70%	Sigma Aldrich	438073 SIGMA-ALDRICH
Gold(III) chloride trihydrate >99.9% trace metal basis	Sigma Aldrich	520918 ALDRICH
1-octanethiol >98.5%	Sigma Aldrich	471836 ALDRICH
Sodium Borohydride purum p.a.>96%	Sigma Aldrich	71320 ALDRICH
addition funnel	SIgma Aldrich	Z330655 SIGMA
Funnel	DURAN	21 351 46
2V folded filtering papers	Whatman	1202-150
Amicon filters	Merck	UFC903024
Iodine, ACS reagent, >99.8%, solid	Sigma Aldrich	207772 SIGMA-ALDRICH
5 mm NMR-Tubes, Type 5HP (high precision)	Armar	32210.503	Length 178 mm
Methanol-d4 99.8 atom%D	Armar	16400.2035
TGA crucible	Thepro	9095-9270.45
400 mesh carbon supported copper grid	Electron Microscopy Science	CF400-Cu
quartz cuvette	Hellma Analytics	100-1-40